A. Field of the Invention
The present invention relates to refrigeration or air conditioning systems using a ternary refrigerant and having a cycle comprising an expander which expands the refrigerant from a liquid phase to a liquid phase and vapor phase. Specifically, the present invention provides a method and system for determining the composition of a ternary refrigerant in a novel and elegant manner.
Chlorofluorocarbon refrigerants, such as dichlorodifluoromethane ("R-22"), present a threat to the earth's ozone layer. Accordingly, practitioners in the air conditioning and refrigeration art have long sought refrigerant compositions which are as effective as chlorofluorocarbon refrigerants, but which are also safe for use in the environment. This search has led to the development of ternary refrigerants, such as "R-407C", a ternary mixture of difluoromethane ("R-32"), pentafluorethane ("R-125") and 1,1,1,2-tetrafluoroethane ("R-134a") combined in a weight ratio of 23/25/52, with properties similar to R-22. The abbreviations R-407C, R-134a, R-125, R-32, R-22, and many others, are designated by the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) and are used throughout the industry and in the present disclosure.
The use of ternary refrigerants poses certain problems. Refrigeration systems periodically leak, which may cause fractionation, and a consequent change in composition of the refrigerant. One problem posed by the use of ternary refrigerants has been the inability to determine whether and the degree to which a ternary refrigerant has fractionated.
Measuring the composition of a ternary refrigerant, such as R-407C, is more complex than measuring the composition of a binary refrigerant because each component adds a degree of freedom to the thermodynamic equation of state, requiring the input of more process variables to reach a solution. A simple method of determining the composition of a ternary refrigerant would greatly facilitate the use and application of new ternary refrigerants.
B. Description of the Prior Art
Methods are known wherein thermodynamic properties are measured, and equations of state are used to calculate the composition of a refrigerant. For example, U.S. Pat. No. 5,626,026 to Sumida describes a complex process for control of a refrigeration system. Sumida describes a system to calculate the composition of a refrigerant, to control the compressor and expansion valve in a refrigeration system. However, the process described in the Sumida is not directly applicable to ternary refrigerants.